Rectangular bales are able to maintain their shape by means of a series of parallel extending twine loops, provided lengthwise around the bales by means of a tying mechanism. Balers typically use a tying mechanism including automatic knotters by which e.g. two knots are made on every loop for binding a bale. An example of such a double knotter system for a baler is disclosed in US 2006/0012176 and in BE 2012/0697 in the name of the Applicant. The knotters are triggered to make a knot when a bale that is being formed in the bale chamber of the baler has reached a predetermined length. For measuring the length of the bale in the bale chamber length measurements are performed.
At present bale length measurements are performed using a star wheel engaging the crop material of a bale in the bale chamber. However, the parameters of the crop material, such as density, humidity, crop type, etc, may influence the amount of slip occurring as the star wheel rotates. Furthermore, depending on the parameters of the crop material, the crop material has the tendency to expand after compression. The slip of the star wheel and expansion of the crop material may lead to inaccurate measurement results.